Coating compositions comprising organic solvents are dangerous to use, being inflammable and liable to explode, and are physiologically and environmentally harmful. Aqueous coating compositions are therefore, in increasing demand. However, in order to dissolve or disperse a suitable resin in water, it is necessary that the resin should carry a number of hydrophilic groups, to use a neutralizing agent capable of forming a water-soluble salt with the resin, and to use a resin of relatively lower molecular weight. As a result, the performance of films applied using aqueous coating compositions e.g. their durability and water-resistance, tends to be lower than would be desirable. Further, aqueous coating compositions cannot have a high non-volatile content, and their working properties are therefore often unsatisfactory. Again, it is often difficult to disperse colouring matter in aqueous coating compositions, thereby limiting their use, and often excluding fields requiring a high order of finishing appearance, gloss and sharpness, and other properties which are required in, for example, automobile industry.
In general, emulsion coating compositions including a resin prepared by the so-called emulsion polymerisation in an aqueous medium, can include relatively high molecular weight resins. The polymerisation technique must, however, be very carefully controlled. However, the surfactant or emulsifier which is added can reduce the durability and water-resistance of the film which is ultimately obtained.
Powder coating compositions have been suggested as an alternative to coating compositions including an organic solvent, but again problems arise in their use. Blocking may occur if the glass transition point of the resin is too low, giving a coating of poor film properties, and the need to have a high baking temperature and a special applicator limits their field of use.
Coating compositions comprising a powder dispersed in water, as a slurry, are also known. Such compositions cannot have a high non-volatile content, making it difficult to obtain a thick coating and/or satisfactory film adhesion (which can lead to cracks in the film). Dispersed powder coating compositions are difficult to apply by spraying, and tend to have low gloss. The slurried powder tends to separate out during storage and, since a dispersing agent is usually present, the water-resistance of the films obtained is often low. Slurry compositions have not generally been used in practice. Many attempts have been made to overcome the problems associated with aqueous dispersion type coating compositions. In particular, a water-soluble or water-dispersible resin have been added; for example, Japanese Patent Applications Nos. 127151/74, 25224/76 and 31636/76 disclose compositions comprising a water-insoluble, dispersed resin as the main ingredient, with a minor amount of a water-soluble or water-dispersible resin. Japanese Patent Applications Nos. 74606/79 and 170262/79 disclose coating compositions comprising a pulverised coating resin and a carboxyl group-bearing water soluble resin in a weight ratio, as solids, of from 0.1 to 100:1; in the specific examples, however, the content of water-soluble resin is about 10 to 20% of the powdered resin and is thus no more than a dispersion stabilizer in a dispersion system. The same is true with respect to the disclosure of Japanese Patent Publication No. 4149/80, of an aqueous dispersion coating composition comprising at least two water-insoluble resinous powders and one or more water-soluble resins.
In using an aqueous dispersion coating medium, the dispersion stability of the powder is always a problem. Various attempts have therefore been made to use finely pulverised resins or dispersion aids. However, since the known systems have been developed on the assumption that, if the powder is surrounded by a water-soluble resin, dispersion stability is improved owing to the affinity of the water-soluble resin to the aqueous medium, it is not surprising that the known compositions comprise only a low content of the water-soluble resin with respect to the dispersed resin.